/*
You are given two non-empty linked lists representing two non-negative integers. 
The digits are stored in reverse order and each of their nodes contain a single digit. 
Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/add-two-numbers
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

#include <math>

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *l3 = new ListNode(0);
        l3->next = NULL;
        
        ListNode *l3_begin = l3;

        int carry_bit = 0;

        while (l1 || l2 || carry_bit != 0)
        {
            int sum = carry_bit;
            if (l1 && l2)
            {
                sum = l1->val + l2->val + carry_bit;
            }
            else if (l1)
            {
                sum = l1->val + carry_bit;
            }
            else if (l2)
            {
                sum = l2->val + carry_bit;
            }
            
            int n1 = floor(sum / 10);
            int n2 = sum % 10;

            carry_bit = n1;
            l3->val = n2;

            if (l1)
            {
                l1 = l1->next;
            }
            if (l2)
            {
                l2 = l2->next;
            }

            if (l1 || l2 || carry_bit != 0)
            {
                l3->next = new ListNode(0);
                l3 = l3->next;
            }
            else
            {
                l3->next = NULL;
            }
        }

        return l3_begin;
        
    }
};
